Detection of infrared (IR) signals and images has important scientific, commercial, and military applications. Advances in the last few decades have allowed extraordinary capabilities in IR sensing and thermal imaging. Further enhancements in resolution and sensitivity will enhance these capabilities. Images of IR-emitting objects can be formed by suitable optical elements onto a Focal Plane Array (FPA) or Staring Array. This is an array of detectors, usually rectangular, each of them corresponding to a pixel. For visible light the most common type of FPAs are the Charge-Coupled Device (CCD) and the Active Pixel Sensor (APS), which are based on silicon technology and are not useful in the IR range. For IR image acquisition with a FPA each pixel must be an IR-sensitive detector. In general, IR detectors can be classified as quantum detectors or thermal detectors. Quantum detectors, which rely on electron transitions in semiconductors, are efficient and can be manufactured in very small dimensions. However, since electron transitions can be caused by ambient thermal energy in the narrow-band semiconductors required for this application, IR quantum detectors require cooling during their operation, often to cryogenic temperatures (˜77 K), in order to avoid thermal noise. This requirement is a serious obstacle in many applications for which cryogenic cooling cannot be provided.
Thermal detectors for IR radiation do not require cryogenic cooling and have the further advantages of very wide spectral response and nearly flat responsivity as a function of wavelength of the incident radiation. On the other hand, they have much lower detectivity than quantum detectors, and are generally not effective in very small sensing areas, which puts a lower limit on the device size and hence the pixel size possible for IR arrays based on thermal detectors. Therefore, development of highly sensitive uncooled IR detectors and arrays, which do not require cryogenic cooling, has substantial interest; since these allow simpler device fabrication, lower operation costs, and facilitates implementation in many settings. Uncooled thermal detectors and arrays for the IR are well known in the art and have been used for many years. In the most common types, the increased temperature at an IR absorbing surface is sensed by means of a thermopile, a thermistor, or a pyroelectric material, but more recently the mechanical flexure of a cantilever caused by absorbed IR energy is also being used in uncooled detectors.
In the last 15 years uncooled thermal detectors employing cantilevers or cantilever arrays have been developed. These employ the fact that a bi-material or “bimorph” cantilever will bend as its temperature changes due to differential thermal expansion of the two materials. Detection of cantilever bending by means of electrical response of the sensing material itself or, more commonly, by other mechanisms which sense cantilever motion, have been implemented using piezoresistance, field emission devices, thermopiles, pyroelectricity, and resistance.
However, thermal detectors that rely on changes of electrical properties of the sensing material or of structures coupled to the sensing material still require electrical connections to each pixel, along with the electronics to read and display the sensed information. The use of uncooled detectors which can be read by direct optical means provides further advantages in terms of simplicity of fabrication and implementation. Direct optical readout methods have been demonstrated for FPAs in which microcantilever array elements are deformed by the absorbed IR radiation and these in turn reflect or scatter visible light, which can be viewed directly by the eye or imaged onto a CCD. A similar principle is used for a different purpose in a proposed IR optical limiter based on microcantilevers. A technique employing evanescent wave coupling in planar optical waveguides joined to bimorph cantilevers has been proposed as well in order to detect their bending.
The sensitivity of the cantilever as an IR detector is proportional to both the square of the length of the cantilever and to the difference in thermal expansion coefficients of the two materials in the bimorph. Therefore, cantilevers lose sensitivity rapidly as their length is shortened. Also, the temperature increase ΔT at the pixel itself for a given net absorbed IR flux (energy per unit area per unit time) qnet is ΔT=qnet A/G, where A is the pixel area and G is the thermal conductance, which includes contributions due to conduction, convection, and radiation losses. Therefore, other things being equal, a device sensitive to a smaller ΔT for a given absorbing area can actually be designed with smaller pixels, and hence can achieve higher image resolution. Currently, however, cantilever lengths in IR FPAs are limited to lengths of ˜100 μm or more. While cantilevers can be easily fabricated in sizes of order 10 to 100 μm with current surface micromachining and standard contact lithography technologies, and in fact there is no technological obstacle to fabrication of cantilevers with sub-micron dimensions, it is the working principle of these devices that in practice places a lower limit on the size of a useful cantilever. It must be stressed that in all the cases known in the art the actuating principle for the microcantilever-based FPAs, whether for electrical or direct optical detection, is the differential thermal expansion coefficients of the two materials in the cantilever structure. In fact, in order to obtain the maximum deflection per temperature degree, the materials that form the “bimorph” cantilever need to be chosen so they have a large enough difference in thermal expansion coefficients, which adds a constraint on the materials that can be used. Actuation based on differential thermal expansion leads to a fundamental materials property restriction which places a lower limit on the size of the detectors and hence the size of the pixels which the FPA can usefully have. The present invention addresses this restriction and makes it possible to design and fabricate microcantilever-based FPAs with substantially reduced pixel sizes.